Generally, an image forming apparatus such as copier, facsimile or printer is equipped with a paper output device for discharging and conveying a sheet of paper that has undergone an image forming process onto a paper output tray. And, in many cases, such a paper output device is provided with a fullness detection sensor to detect fullness of sheets of paper on the paper output tray. It has been considered that, because the fullness of the paper on the paper output tray is detected using the fullness detection sensor, such a malfunction as the paper keeps being successively outputted while the paper output tray is in fullness can be prevented from occurring, and thus that the malfunction related to output of the paper by the paper output device can be prevented from occurring.
However, the paper on the paper output tray is not always kept flat, but sometimes can be curved upward at its both edges in paper width direction perpendicular to a paper conveying direction, or at its rear edge. For example, the paper such as recording paper having been sent out of a fuser unit can be left with a state of curling in which both edge portions of the paper in the paper width direction are curved upward due to an effect of heating at the time of fusing.
Thus, among conventional image forming apparatus is there one that is configured so as to have, both in the middle and in the neighborhoods of the edge portions in the paper width direction, a plurality of fullness detection means each to detect an occurrence of the fullness of the paper having been outputted on the paper output tray (for example, refer to Patent Literature 1). In the technique, a method of detection is employed in which a position of an actuator for fullness detection and a position of another actuator for curling detection are detected by a single photo sensor.